The digital memory chip most commonly used in computers and computer system components is the dynamic random access memory (DRAM), wherein voltage stored in capacitors represents digital bits of information. Electric power must be supplied to the capacitors to maintain the information because, without frequent refresh cycles, the stored charge dissipates, and the information is lost. Memories that require constant power are known as volatile memories.
Non-volatile memories do not need frequent refresh cycles to preserve their stored information, so they consume less power than volatile memories. The information stays in the memory even when the power is turned off. There are many applications where non-volatile memories are preferred or required, such as in lap-top and palm-top computers, cell phones or control systems of automobiles. Non-volatile memories include magnetic random access memories (MRAMs), erasable programmable read only memories (EPROMs) and variations thereof.
Another type of non-volatile memory is the programmable conductor or programmable metallization memory cell, which is described by Kozicki et al. in (U.S. Pat. Nos. 5,761,115; 5,914,893; and 6,084,796) and is incorporated by reference herein. The programmable conductor cell of Kozicki et al. (also referred to by Kozicki et al. as a “metal dendrite memory”) comprises a glass ion conductor, such as a chalcogenide-metal ion glass, and a plurality of electrodes disposed at the surface of the fast ion conductor and spaced a distance apart from one another. The glass/ion element shall be referred to herein as a “glass electrolyte” or, more generally, “cell body.” When a voltage is applied across the anode and cathode, a non-volatile conductive pathway (considered a sidewall “dendrite” by Kozicki et al.) grows from the cathode through or along the cell body towards the anode. The growth of the dendrite depends upon applied voltage and time; the higher the voltage, the faster the growth rate; the longer the time, the longer the dendrite. The dendrite can retract, re-dissolving the metal ions into the cell body, by reversing the polarity of the voltage at the electrodes.
In the case of a dielectric material, programmable capacitance between electrodes are programmed by the extent of dendrite growth. In the case of resistive material, programmable resistances are also programmed in accordance with the extent of dendrite growth. The resistance or capacitance of the cell thus changes with changing dendrite length. By completely shorting the glass electrolyte, the metal dendrite can cause a radical change in current flow through the cell, defining a different memory state.
For the proper functioning of a memory device incorporating such a chalcogenide-metal ion glass element, it is important that growth of the conductive pathway have a reproducible relationship to applied voltage. For device operation, multiple cells across an array should ideally have a consistent response to the signals they receive.
The current invention addresses the issue of consistent memory cell response by ensuring a uniform supply of metal ions for formation of a conductive pathway under applied voltage.